1. Technical Field
Exemplary aspects of the present invention relate to a fixing device, an image forming apparatus, and a fixing method, and more particularly, to a fixing device for fixing an image on a recording medium, an image forming apparatus incorporating the fixing device, and a fixing method for fixing a toner image on a recording medium.
2. Description of the Background
Related-art image forming apparatuses, such as copiers, facsimile machines, printers, or multifunction printers having two or more of copying, printing, scanning, facsimile, plotter, and other functions, typically form an image on a recording medium according to image data. Thus, for example, a charger uniformly charges a surface of a photoconductor; an optical writer emits a light beam onto the charged surface of the photoconductor to form an electrostatic latent image on the photoconductor according to the image data; a development device supplies toner to the electrostatic latent image formed on the photoconductor to render the electrostatic latent image visible as a toner image; the toner image is directly transferred from the photoconductor onto a recording medium or is indirectly transferred from the photoconductor onto a recording medium via an intermediate transfer belt; finally, a fixing device applies heat and pressure to the recording medium bearing the toner image to fix the toner image on the recording medium, thus forming the image on the recording medium.
The fixing device may employ an endless belt or an endless film to heat the recording medium. For example, the fixing device may include an endless belt and a pressure roller pressed against the belt to form a fixing nip therebetween. As a recording medium bearing a toner image is conveyed through the fixing nip, the belt and the pressure roller apply heat and pressure to the recording medium, melting and fixing the toner image on the recording medium.
The belt is requested to be heated quickly to shorten a first print time taken to output the recording medium bearing the fixed toner image upon receipt of a print job. Additionally, as the image forming apparatus conveys an increased amount of recording media at high speed, the belt is requested to overcome shortage of heat.
In order to shorten the first print time, the fixing device may include an endless film and a pressure roller pressed against a ceramic heater disposed inside the film to form a fixing nip between the film and the pressure roller. As a recording medium bearing a toner image is conveyed through the fixing nip, the film heated by the ceramic heater and the pressure roller fix the toner image on the recording medium under heat and pressure. Since the film is heated by the heater situated at the fixing nip, the film is heated insufficiently at an entry to the fixing nip, resulting in faulty fixing. Accordingly, the film is requested to overcome shortage of heat at the entry to the fixing nip.
To address those requests, the fixing device may employ a tubular, metal thermal conductor disposed inside the belt. The pressure roller is pressed against the metal thermal conductor via the belt to form a fixing nip between the belt and the pressure roller. A heater situated inside the metal thermal conductor heats the metal thermal conductor which in turn heats the belt. As a recording medium bearing a toner image is conveyed through the fixing nip, the belt heated by the metal thermal conductor and the pressure roller apply heat and pressure to the recording medium, fixing the toner image on the recording medium. Since the tubular, metal thermal conductor is disposed opposite the belt throughout the entire circumferential span thereof, the metal thermal conductor heats the belt quickly, thus shortening the first print time and overcoming shortage of heat.
In order to shorten the first print time and save energy further, the belt heated by the heater directly, not through the metal thermal conductor, is proposed. For example, a heater disposed inside the belt heats the belt directly. A shield plate is interposed between the heater and the belt to shield the belt from the heater. A controller moves the shield plate based on the size of the recording medium to change a direct heating area where the heater heats the belt directly. The shield plate includes a slot through which the heater irradiates the belt directly and a shield portion that shields the belt from the heater. The controller moves the shield portion to change the width of the slot, thus changing the direct heating area where the heater heats the belt directly.
As a fixing job finishes, the heater is turned off and therefore the belt stops heating the recording medium. However, residual heat is conducted from the heater to the shield plate. Since the belt stops rotation as the fixing job finishes, the heated shield plate may heat the belt locally, causing temperature variation of the belt in a circumferential direction thereof that may result in local plastic deformation of the belt such as kink. To address this problem, when the belt overheats locally, the belt may resume rotation to level residual heat conducted to the belt in the circumferential direction thereof so as to reduce temperature variation thereof.
However, even when the belt resumes rotation, the shield plate shielding the belt from the heater may be heated by residual heat from the heater, obstructing leveling of residual heat conducted to the belt. Additionally, when the shield plate is heated, the shield plate may halt at an inappropriate position where heat is not conducted from the shield plate to a peripheral component. Conversely, when the shield plate is cool, the shield plate may draw heat from belt locally, causing temperature variation of the belt in the circumferential direction thereof.